The second returned value, which is one of the symbols
:simple, :complex, or nil, indicates which type of clipping region the
window currently has. :simple means that box
returned as the first value exactly describes the clipping
region. :complex means that the box encompasses a
non-rectangular clipping region; Common Graphics does not provide any
further information about this region however. nil means that the clipping region is empty (of size
zero) and so nothing can be drawn on the stream currently; in this
case the first return value is the box (0 0 0 0), though the arbitrary
"position" of this box at (0 0) is not meaningful.

Note that the box returned (as the first value) is not merely the
value that may have been set by a call to (setf clipping-box) or
with-clipping-box; it also reflects
how the window is partly or fully covered by other windows and how it
is scrolled. Specifically, the returned box is the smallest box
containing the currently visible parts of the window's interior (in
stream units), intersected with the box most recently specified by a
call to (setf clipping-box) or with-clipping-box, if any. A window
whose interior is completely covered will always return the box (0 0 0
0) and a second value of nil to indicate a currently empty clipping
region.

An application may call (setf clipping-box) at any time to set
the current clipping-box of a
Common Graphics stream. The value argument may be either a box to
establish clipping, or else nil to remove all
clipping. The macro with-clipping-box is a handy alternative because
it guarrantees that the original clipping-box will be restored after
its body executes.

While the function (setf clipping-box) and the macro with-clipping-box are
often useful for establishing a clipping region, applications
generally are not expected to have a need to call clipping-box or nclipping-box to retrieve the
current region. The functions are not reliable for recalling the
region that was explicitly set, since the returned box is also
affected by how the window is covered. Furthermore, all drawing code
for a window (other than a bitmap-pane) normally should be
placed within a redisplay-window method, which
always receives a box argument denoting the area that needs to be
redisplayed, and the clipping-box of the window is set to this box
automatically during the call to redisplay-window.

Platform note: the box that is returned will exclude any part
of the window that happens to be covered by other windows on the
Microsoft Windows platform, but will not exclude these areas on GTK.
Also, the second return value will never be :complex on GTK.